Seeing the light 57
They couldn’t even distinguish colors whose
differences are patently obvious to our eyes.
So why all the receptors? Thoen suspects that
they have everything to do with pugilistic prow-
ess. We do a lot of visual processing in our retinas,
adding and subtracting information from our
cones before sending it to our brains. Perhaps
the mantis shrimp instead passes the responses
of all 12 of its color receptors directly up to its
brain, which compares the raw data against
some kind of look-up table of different colors.
While the mantis shrimp is inept at discriminat-
ing between colors, such a system might make
it superb at recognizing color, which in turn
could help it make the quick decisions needed
to launch its superlatively fast strikes.
But Cronin is unconvinced. Back in his lab,
he dangles a pipette in a petri dish containing a
smaller mantis shrimp—just a couple of inches
long. It tracks the intruding object with its eyes,
then lashes out. The blow is powerful enough to
make an audible crack, like a finger snapping.
“That little guy spent a long time thinking be-
fore he whacked it. It’s not a decision they make
like that,” says Cronin, snapping his own fingers.
“There remains the question: What’s it all for?”
It’s the question that Dan-Eric Nilsson always
asks as well. It’s not enough to know the struc-
ture of the mantis shrimp’s eyes, or the genes
that are activated within them, or the neural
signals that they send to the brain. Ultimately,
to understand why they are the way they are, we
need to know how they are used. To communi-
cate with each other? To catch prey quickly? To
better see the riot of colors in coral reefs? This
is the ultimate truth of animal eyes: We can only
understand their evolution when we learn to see
the world through them. j
The biggest eyes belong to the
largest squid species. The eye of
a giant squid (Architeuthis dux) at
right is six and a half inches across;
others up to a foot are known. They
may allow the animal to make out
the twinkle of glowing plankton
disturbed by charging sperm
whales—the squid’s archnemesis.
PHOTOGRAPHED AT NATIONAL MUSEUM OF NATURAL
HISTORY, SMITHSONIAN INSTITUTION
Whose eyes are these?
1 Southern ground-hornbill (Bucorvus leadbeateri); 2 White rhinoceros (Ceratotherium
simum); 3 Gargoyle gecko (Rhacodactylus auriculatus); 4 Bobcat (Lynx rufus); 5 African gray
parrot (Psittacus erithacus); 6 Red-eyed tree frog (Agalychnis callidryas); 7 Scarlet macaw
(Ara macao); 8 Dumpy tree frog (Litoria caerulea); 9 Rainbow lorikeet (Trichoglossus haem-
atodus haematodus); 10 Agassiz’s desert tortoise (Gopherus agassizii); 11 Common ostrich
(Struthio camelus); 12 Blue-eyed black lemur (Eulemur flavifrons); 13 Mossy leaf-tailed gecko
(Uroplatus sikorae); 14 Domestic dog (Canis lupus familiaris); 15 Western lowland gorilla
(Gorilla gorilla gorilla); 16 Green tree python (Morelia viridis); 17 Human (Homo sapiens);
18 Panther chameleon (Furcifer pardalis); 19 Common boa (Boa constrictor imperator)
1
6
11
16
2
7
12
17
3
8
13
18
4
9
14
19
5
10
15